The present invention generally relates to point-to-point high frequency, high data rate wireless telecommunication systems and, more particularly, to band efficient communications using polarization division duplexing.
There is an increasing need for very high data rates, over 300 Mbps (megabits per second), in wireless telecommunication systems. High data transmission rates are needed, for example, in backhaul networks, LMDS (local multipoint distribution service), MMDS (multichannel or microwave multipoint distribution service), dedicated links in campus networks, private communication networks, and high speed internet services such as cable modems and DSL (digital subscriber line). In areas without gas or steam pipes or other underground conduits, it is less costly to set up wireless transceivers on rooftops than to dig up the ground to install optical fiber. Installation of a wireless link requires only the placement of antennas and associated equipment at the desired locations and can be completed and functional within a day as opposed to the several days, or longer, required to install a hardwired system. There may be additional advantages if the antenna and hardware that comprise a terminal are made small enough in size to be mobile for use in temporary setups supporting special events such as sporting events or concerts. Thus, there is increasing demand for high performance, high data rate links to support system architectures transmitting at OC-3 (optical carrier 3, with a data rate of 155 Mbps) to OC-96 (4.9 Gbps, or gigabits per second). Current wireless systems supporting high data rate networks achieve up to approximately 600 Mbps (OC-12).
Capability of current wireless systems is limited due to smaller channel bandwidth allocations, typically less than 100 MHz (mega hertz, or million cycles per second), available in the Federal Communications Commission (FCC) allocated frequency bands in the range of 1 to 40 GHz. One approach to providing increased data rates is to use a higher frequency range, for example, 70 to 100 GHz, where there is currently much lower usage and, therefore, potential availability for wider channel bandwidth allocations. Even at higher frequencies and with wider channel bandwidth allocations, however, there still exist economic, technical, and other incentives to make the most efficient use of the allocated channel bandwidth because, for example, of the limited availability of such channel bandwidth allocations, their licensing cost, and operating costs to use them. As an example, one way to achieve higher data rates within a given channel bandwidth allocation, is to use modulation schemes that are more bandwidth efficient, by increasing the modulation order, which allows higher data rates to be used. Achieving more bandwidth-efficient modulation presents several technical challenges, however, and typically requires more complex and expensive hardware.
Frequency reuse schemes are another approach to achieving greater bandwidth efficiency. In a frequency reuse scheme, different signals may be transmitted over the same frequencies. In order to be able to separate the different signals from each other, the signals are made orthogonal—thus seperable—by the use, for example, of techniques such as polarization, code division multiplexing, or time division multiplexing. The frequency reuse approach can effectively increase data rates by transmitting more data on separated signals over the same frequencies.
One example of frequency reuse in current radio frequency (RF) wireless communication systems is the use of linear polarization of the RF carrier signal waveforms. When using linear polarization for frequency reuse, the varying electromagnetic fields of the RF carrier signal waveforms are transmitted in two orthogonal planes of polarization, which typically may be oriented in the vertical or horizontal directions. Then, for example, one set of data may be transmitted in a horizontal plane of polarization while a second, independent set of data is transmitted over the same RF carrier frequency in a vertical plane of polarization. By the implementation of such a frequency reuse scheme using linear polarization, the amount of data that can be communicated in a given bandwidth may be potentially doubled without otherwise increasing the data rate or changing the modulation scheme. In this example using linear polarization for frequency reuse, the receiving antennas convert the varying electromagnetic fields of the RF carriers to electronic waveforms. In order for the conversions to be accurate, linear polarization requires close alignment of the axes of the transmit and receive antennas to match the plane of polarization. Any unintentional rotation of one antenna with respect to the other may result in significant received power losses. Another problem in frequency reuse schemes using polarization is attenuation and depolarization due to the transmission of the signals through rain or other atmospheric conditions.
Another example of frequency reuse in current radio frequency (RF) wireless communication systems is the use of circular polarization of the RF carrier signal waveforms. When using circular polarization for frequency reuse, the varying electromagnetic fields, or waves, of the RF carrier signal are excited and transmitted with components in two orthogonal coordinates where, for example, one coordinate may be horizontal and the other may be vertical. The orthogonal components are combined so that the combinations will produce an electromagnetic wave that rotates circularly as the RF carrier signal propagates. Depending on the direction of rotation of the circularly rotating electromagnetic wave, either a right-hand circular polarization (RHCP) or a left-hand circular polarization (LHCP) is produced. Then, for example, one set of data may be transmitted using LHCP while a second, independent set of data is transmitted over the same RF carrier frequency using RHCP. By the implementation of such a frequency reuse scheme using circular polarization, the amount of data that can be communicated in a given bandwidth may be potentially doubled without changing the data rate or the modulation scheme. Axial alignment of the antennas, i.e. pointing the antennas directly at each other, improves received power, but unlike linear polarization, circular polarization does not require rotational, or radial, alignment of the polarization axes of the transmit and receive antennas to match a plane of polarization, so that rotation of one antenna with respect to the other does not affect received power.
U.S Pat. No. 5,905,574, issued May 18, 1999, entitled “Method and Apparatus for Canceling Cross Polarization Interference”, and assigned to the assignee of the present invention, describes a system for dual signal transmission with a circular polarization frequency reuse scheme transmitting two RF carrier signals at the same frequency propagating in the same direction, one signal using RHCP and the other signal using LHCP. The circular polarization frequency reuse scheme introduces interference between the two RF carrier signals due to cross-polarization coupling of the two RF carrier signals. In order to reduce this cross-polarization coupling, an adaptive interference reduction circuit may be integrated into the circular polarization frequency reuse scheme. U.S. Pat. No. 5,905,574 does not disclose a full duplex system, i.e., a system for two-way communication; the adaptive interference reduction circuit integrated into the circular polarization frequency reuse scheme enhances communication in one direction only.
Frequency division duplexing (FDD) is the most common method of sending and receiving a full duplex signal, i.e., communicating in two directions simultaneously. FDD distinguishes between transmit and receive signals by placing them on two different frequencies. The antenna will transmit a signal on one frequency and receive another signal on a different frequency. The FDD full duplex method could require, for example, twice as much bandwidth as single duplex communication because forward transmission occurs on one channel, using a certain amount of bandwidth, and return transmission occurs on another channel, using additional bandwidth. For a telecommunication system that transmits and receives the same amount of data at similar data rates, two channels of approximately the same bandwidth are needed.
By implementing a full duplex system using a frequency reuse scheme, significant gains in channel bandwidth economy may be achieved. For example, a full duplex system, i.e., a system for two-way communication, might be implemented over a single center frequency, or channel, by transmitting a forward link on one polarization and a return link on another, orthogonal, polarization. Circular polarization has several clear advantages over linear polarization for the implementation of such a frequency reuse scheme. For example, circular polarization is less susceptible than linear polarization to interference and depolarization due to the transmission of the signals through rain or other atmospheric conditions. Also, rotational misalignment of the antennas is less a problem with circular polarization than with linear polarization, as described above. There is an interference problem with circular polarization, however, due to reflections of the circularly polarized electromagnetic waves returning to the transmit/receive antenna on the opposite polarization. Electromagnetic waves may be reflected, for example, from the surfaces of buildings or from foliage. If an RHCP electromagnetic wave is reflected it will be reflected as an LHCP electromagnetic wave, and vice versa. So, for example, if RHCP signals are being transmitted and LHCP signals are being received, a reflected RHCP transmit signal returns as LHCP as a result of being reflected so that it interferes with the LHCP receive signal. Thus, the transmit signal would undesirably interfere with the receive signal at the transmitter/receiver.
As can be seen, there is a need for a full duplex telecommunication system that effectively takes advantage of frequency reuse using circular polarization. There is also a need for a mechanism that will detect whether or not a circularly polarized electromagnetic wave being received is in fact the proper signal coming into the receiver or an undesirable reflection of an electromagnetic wave transmitted from the same site on the opposite circular polarization.